President Donald Trump might soon toss the Iran nuclear deal back to Congress to decide its fate — but it’s no sure thing that lawmakers will kill it for good.

Critics of the Obama-era agreement, led by the president himself, appear to be on a mission to tank the accord. Naysayers in Congress are vowing to take advantage of a fast-track mechanism that would allow lawmakers to reinstate sanctions against Tehran with a simple majority vote.

But while congressional Republicans unanimously opposed the nuclear deal two years ago, there’s far less unity on how quickly the Republican-led Congress should move to “snap back” sanctions that were lifted as part of the Iran agreement. Doing so would effectively dismantle the 2015 deal.

Even a few defections would make a difference in the Senate, where Republicans hold a narrow 52-seat majority. Democratic senators, even those who opposed the nuclear agreement two years ago, want the deal to remain in place. As it stands now, enough Republicans are undecided that party leaders would struggle to corral the votes needed to reimpose sanctions.

The choice could land on Capitol Hill later this month, if Trump declines to certify by an October 15 deadline that Iran is complying with the nuclear deal. The agreement lifted stiff sanctions against Iran that had been in place for decades in exchange for significant restrictions on the nation's nuclear program.

“I believe we should abrogate the agreement immediately, reimpose sanctions and use every diplomatic and economic lever we have to shut down Iran’s nuclear program" — Ted Cruz

Senator Jeff Flake (R-Ariz.) made clear he’s undecided on whether to reimpose sanctions, saying in an interview that “we’ll have to look and see what that does.”

“I don’t think that we should relieve Iran of its obligations,” said Flake, who sits on the foreign relations committee. “They realize the benefits already of the sanctions relief. And now, to be in a position where they could get out from under the protocols under the agreement, that’s what I’m worried about.”

Senator John McCain (R-Ariz.) told reporters recently that he is “still looking at” the issue and remains undecided. Senator Susan Collins (R-Maine) said she wants to see the evidence on whether Iran has been compliant with the deal or not.

Other influential players, such as Senator Rand Paul (R-Ky.) and House foreign affairs committee Chairman Ed Royce (R-Calif.), don’t believe Trump should begin retreating from the deal. Royce said the president should instead “enforce the hell out of” it.

Senate Democrats overwhelmingly oppose withdrawing from the agreement, even though several of them rejected it two years ago, including now-Minority Leader Chuck Schumer of New York and Maryland Senator Ben Cardin, the top Democrat on the foreign relations committee.

“There is a clear cause-and-effect relationship: Republicans would cast a vote that would set Iran back on a path to a nuclear weapon,” Senator Chris Murphy (D-Conn.) said. “I just can’t understand why any Republican would want that on their conscience, or would want that politically.”

Congress has other options besides reimposing sanctions that were lifted as part of the 2015 deal. Senate Majority Leader Mitch McConnell could decide to move a new sanctions plan, though that strategy would require 60 votes, which could make it considerably more difficult for the Kentucky lawmaker to move such a measure through the chamber.

Lawmakers could also do nothing. Even if Trump declares Iran to be in violation of the agreement, de-certifying by itself wouldn't kill the nuclear deal, Iranian Foreign Minister Javad Zarif pointed out in a recent interview with POLITICO.

But quickly reimposing sanctions is the most appealing option for some Republicans pushing for a tougher policy toward Tehran. It would take only a simple majority in both chambers of Congress, as long as lawmakers do so within 60 days of Trump declining to certify that Iran was complying with the deal.

"I believe we should abrogate the agreement immediately, reimpose sanctions and use every diplomatic and economic lever we have to shut down Iran's nuclear program," Senator Ted Cruz (R-Texas) said in an interview.

Other influential Republican hawks have been back-channeling with key administration figures over the fate of the nuclear deal. Arkansas Senator Tom Cotton, who will deliver a major speech Tuesday on Iran, has been speaking with top White House officials about next steps on U.S. strategy toward the nation, including at a meeting Wednesday with National Security Adviser H.R. McMaster and other National Security Council officials.

Cotton is making the case that even if Iran is technically complying with the terms of the nuclear agreement, Trump shouldn’t certify anyway because doing so is not in the United States’ national interest. Administration officials have argued that Iran has violated the “spirit” of the deal, even as international inspectors say Tehran has complied.

Senate foreign relations committee Chairman Bob Corker also has been in touch with Trump and White House officials about the future of the deal. The Tennessee senator declined to say whether Congress would reimpose sanctions or whether he believes Trump should declare Iran in violation of the deal. “I’m so close to it in talking with them daily that I really can’t speak to the issue,” Corker said.

Trump has hinted heavily that he is preparing to withdraw from the controversial agreement. He declared at the United Nations General Assembly last month that the agreement was an “embarrassment to the United States.” Still, Trump has certified the nuclear deal twice this year.

The deal’s backers warn that a restoration of sanctions by Congress would be a self-inflicted wound. European allies such as France, Germany and Great Britain, which helped to negotiate the agreement, strongly oppose such a move. Even many critics of the deal — which lifted international sanctions in return for strict limits on Iran’s nuclear program — say the moment to oppose it has long passed, given that it unfroze tens of billions of dollars in Iranian assets that cannot be reclaimed.

Amid the uncertainty, Democrats have pushed for more information from the administration about its Iran policy but have so far been rebuffed. Schumer asked the White House for an all-senators briefing on the status of the Iran deal by early October, but the White House offered to do so only after the October 15 deadline had passed, according to a congressional source.

“Once those sanctions are reimposed, the U.S. would be the party violating the agreement, not Iran" — Senior Democratic official

Cardin and six other senior Senate Democrats wrote to five Trump Cabinet officials last month asking for a report on “any information that would suggest that Iran is no longer complying” with the agreement, asking the administration to respond by Friday.

A 2015 law written by Corker and Cardin that set up the congressional review process for the Iran deal requires that lawmakers be notified within 10 days if “any potentially significant Iranian breach or compliance concern related to the” deal was discovered, Democrats wrote in the letter.

But “no such notification” has yet occurred, the senators added.

In an interview, Cardin said, “I don’t understand what the president’s strategy is” toward Iran. He and Corker are scheduled to talk privately about the issue this week.

“I think what he’s doing is a signal to the international community … that the United States is seriously contemplating withdrawing from the nuclear deal, contrary to the agreement, contrary to a U.N. resolution,” Cardin said of Trump. “That makes it extremely difficult for us to get the cooperation we need in dealing with Iran’s non-nuclear violations,” he added, referring to its ballistic missile program and other issues.

Republicans who prefer to remain in the nuclear deal are floating other possible punishments for Iran for its non-nuclear activities.

U.S. Senator Rand Paul | Alex Wong/Getty Images

Royce and his Democratic counterpart on the committee, New York Representative Eliot Engel, last week advanced new bipartisan sanctions on the Iran-backed Hezbollah militia. Paul has said instead of withdrawing from the nuclear agreement, the United States should look at going after Iran’s ballistic missile program.

“I’m not so sure what effect it will have” to reimpose sanctions, Paul said, because the other countries who entered into the nuclear deal — the United Kingdom, France, Russia, China and Germany — will “continue to abide by the agreement.”

Bolstering Paul’s point, those European partners, with the threat of multilateral sanctions hanging over Tehran, are signaling that they would work to prevent any U.S.-only reinstatement of sanctions from hurting their economies.

But given Republicans’ eagerness to deliver on a conservative agenda following the failure of their Obamacare repeal plan, some Democrats are bracing for Iran hawks to push to restore sanctions with a simple majority vote. Senator Cory Gardner (R-Colo.) said he believes Congress would move “relatively quickly” to reimpose sanctions.

“The political pressure could be overwhelming on Republicans,” one senior Democratic official said. “Once those sanctions are reimposed, the U.S. would be the party violating the agreement, not Iran.”